Conventionally, shot peening has been used to improve various metal part surfaces by forming, by example, compressive residual stresses, enhance fatigue strength, harden the workpiece.
More recently, to impede stress corrosion cracking and protect materials in critical applications, such as a nuclear reactor vessel, against such cracking, there is also a technique available to suppress the residual stresses on the surface of a workpiece using cavitation generated by injecting compressed water into water via a nozzle comprising two or more throats.
This technique to improve metal part surfaces, however, has been disclosed as using the collapsing impact force of cavitation. Nevertheless, it has been successfully used and has been confused with a “general water jet”, which has a “cavitating jet” that is injected into the air.
In other words, the use of the “general water jet” has assumed that the surface peening level, for example, introduced residual stress value, improved fatigue strength level, surface hardening grade, etc. is dependent upon the pressure of the water injected. On such an assumption, an expensive high-pressure pump is employed to increase the pump discharge pressure. However, satisfactory treatment capability has remained unattainable from the viewpoint of surface treatment. Furthermore, there have been some other problems awaiting solution. The factors which may govern a cavitation collapsing impact force in the surface modification process are not yet fully understood. Additionally, neither the collapsing impact force of the cavitation bubble nor the cavitation jet's surface treatment effect have been effectively utilized.